Current high bit rate second generation radiocommunication satellites operating in the Ka band offer high transmission capacities, of the order of around one hundred Gbps, thanks to the use of narrow antenna beams, combined with spatial re-use of the frequency resources in a fixed broadband system and thanks to an efficient strategy for adaptive coding and modulation of the transmission channels.
Of the standard frequency re-use schemes, the scheme that corresponds to an allocation of four distinct sub-bands across all of the coverage spots of the multi-beam antenna in accordance with a pattern with four colours is well known. A 4-FR frequency re-use scheme employing four colours divides the whole of the band allocated to the system into four distinct sub-bands of frequencies or four colours, and allows adjacent beams of the satellite transmit or receive antenna to transmit in the various sub-bands on a downlink from the satellite to the ground or to receive in the various sub-bands on an uplink from the ground to the satellite. A 4-FR frequency re-use scheme employing four colours enables use of a constant minimum distance between beams of the same colour and consequently a reasonable compromise to be obtained between the re-use factor of the band and the isolation between the beams.
However, with a fixed number of beams of a geographical coverage, if it is wished to reduce the number of colours, the C/I of frequency re-use determined by the distance between two spots of the same colour is reduced, which reduces spectral efficiency and limits or even prevents any increase in the communication capacity of the system.
To improve the total capacity of the system in a downlink context, i.e. in the transmission spots of the satellite antenna, the paper by O. Vidal et al., entitled “Fractional Frequency Reuse in fixed Broadband High Throughput Satellite systems” published in Proceedings of 31st AIAA International Communication Satellite Systems Conference, Oct. 14-17, 2013, Florence, Italy, describes a solution that increases the bandwidth used per spot for each of the spots of a coverage. This solution exploits fractional frequency re-use (FFR) schemes that are used in terrestrial mobile networks such as WiMAX and LTE networks. The FFR technique applied in a satellite radiocommunication system in the context of the downlink is a frequency re-use technique that covers the standard patterns of colours, i.e. three, four, seven, twelve colours, for example, combining them with more dense frequency re-use schemes within each beam. The paper by O. Vidal describes with no limitation of its general applicability a classic 7-FR re-use scheme employing seven colours combined with a 1-FR scheme of total re-use of one sub-band. In this configuration, the sub-band F0 of the 1-FR scheme and the sub-bands Fi of the 7-FR scheme are in permanent use in their allocation beams but anywhere within the coverage and the ratio C/I observed in the sub-band Fi will be greater than the ratio C/I observed in the sub-band F0 because of a lower re-use factor for the sub-band F0.
To improve the total capacity of the system and in an uplink context, i.e. in the reception spots of the satellite antenna, the paper by F. Meng et al., entitled “Comparison of Frequency Reuse Schemes in OFDMA based Multi-beam Satellite Communications” published in Proceedings of 29th AIAA International Communications Satellite Systems Conference, 28 Nov.-1 Dec. 2011 Nara, Japan describes a solution that increases the bandwidth used per reception spot for each of the reception spots of a coverage in the form of a first configuration and a second configuration.
In the first configuration, a common frequency sub-band is allocated in an inner zone of each spot in accordance with a 1-FR scheme of total re-use of a sub-band, and in combination therewith a classic 3-FR three-colour re-use scheme applied to different sub-bands of the common sub-band is used for the peripheral zones of the reception spots, each peripheral spot having a peripheral zone surrounding its inner zone. Note that this sharing of resources on the uplink between an inner zone and a peripheral zone for each reception spot is made possible by virtue of a geo-location functionality installed in each user terminal enabling it to determine in which spot and in which zone it is located (the inner zone or the peripheral zone).
The second configuration is identical to the first configuration except that, for a given reception spot, not only is the band common to all the spots allocated to its inner zone, but also it is allocated the sub-bands of the peripheral zones of the adjacent spots.
Although the two configurations described make it possible to increase the capacity of the system, this increase remains limited, when the size of the inner zones and of the common sub-band increases, because of the interference generated by the inner zones of the adjacent spots.